1. Gather the players: You will need a minimum of two players and a maximum of however many can comfortably play on the basketball court. Ideally, three to five players work best.

2. Choose the shooting order: Start by determining the shooting order of the players. This can be done by taking turns shooting from the free throw line, with whoever makes the shot first going first, and so on.

3. Clear ball possession: When starting a new game or after each made basket, make sure no one is in possession of the ball before resuming play.

4. Shoot to start: The first player shoots from anywhere within three-point range. If they make it, they score points equal to the value of that shot (two or three points, depending on where it was taken). If they miss, the next player retrieves the ball and takes their turn to shoot.

5. Rebound and continue: If a player misses their shot from behind the three-point arc, they can attempt to rebound their own missed shot immediately. They must shoot again within one step, now worth one point per make. A trapped situation can occur when a player does not make any shots; they will have that position until another player starts his turn.

6. Keep track of scores: Each player’s score accumulates as they make shots in increasing increments (1, 2, or 3 points) until someone reaches or surpasses 21 points.

7. Foul penalty (optional): If a player commits a foul while playing defense (blocking or holding), they take an additional one-point penalty.

8. Reach 21 exactly: Players must reach exactly 21 points to win. For example, if a player has 20 points and makes a two-point shot, their score will revert back to previous total before that shot was taken in this case which is 20.

9. “Win by 2” rule (optional): Players might agree that the winner needs to have a lead of at least two points over the second-highest scorer.

10. Playing with teams (optional): Divide players into pairs or teams and follow the same rules, except players will take alternating turns between teammates, and total team scores will determine the winner or winners.

11. Repeat games (optional): If desired, play several rounds and keep a running tally of each player’s total score across all rounds.

12. Sportsmanship: Remember to play fairly, be supportive of each other, and enjoy the game! 21 is a fun way to practice shooting skills and get some friendly competition going on the court.